Environmental Toxicology ond Chemistry, Vol. 9, pp. 141-149, 1990 Printed in the USA. Pergamon Press pic 001401 0730-7268/90 $3.00 + .00 Copyright @ 1990 SETAC PREDICTION OF CONTAMINANT ACCUMULATION BY FREE-LIVING ORGANISMS: APPLICATIONS OF A SIGMOIDAL MODEL I. LEHR BRISBIN, JR.,* MICHAELC. NEWMAN, SUSANG. McDoWELL and ERIC L. PETERS Savannah River Ecology Laboratory, Aiken, South Carolina 29801 (Received 24 August 1988; Accepted 29 January 1989) Abstract- The accumulation of contaminants by free-living organisms has traditionally been de- termined with permutations of the deterministic model: Ct = Ce(1 - e -kt). However, studies uti- lizing a variety of species and exposure scenarios now suggest that significant deviations may occur from this classic form. In many cases noted to date, these deviations have involved a sigmoidal pat- tern of accumulation. While there may be no one single causal mechanism responsible for the ex- pression of such a pattern in all cases studied, the application of a flexibly shaped Richards sigmoidal model can improve goodness of fit to the data relative to the classic model form. Several examples are presented for use of the Richards model: accumulation of 137Cs by free-living American coots (Fulica americana) and yellow-bellied turtles (Pseudemys scripta), and Hg accumulation by mos- quitofish (Gambusia affinis). Keywords - 203Hg Hg Waterfowl INTRODUCTION The accurate prediction of the pattern and rate of contaminant accumulation by free-living organ- isms is an important aspect of toxicological risk as- sessment. Traditionally, accumulation is described by the model where dC/dt is the rate of change of contaminant concentration over time, Cs is the concentration in an infinite source, Ct is the concentration in the organism at time t, ku is the uptake rate constant and kctis the depuration or elimination rate con- stant. At steady state where Ce is the final contaminant concentration in the organism at steady state or equilibrium. Com- bining Equations 1 and 2 and assuming that ku, kct \ *To whom correspondence may be addressed. Presented at the Symposium on Toxicokinetics, Eighth Annual Meeting of the Society of Environmental Toxicology and Chemistry, Pensacola, Florida, Novem- ber 9-12, 1987. E.L. Peters' current address is Department of Radi- ology and Radiation Biology, Colorado State University, Fort Collins, CO 80523. Turtle Fish Radioecology and Cs remain constant, the integrated form of the uptake model becomes (3) (1) which indicates that the concentration in the whole body or some organ/tissue compartment increases over time at a rate that is maximal at the time of first exposure. The rate of accumulation then gradually decreases over time until some asympto- tic concentration is attained [1-3]. The depuration rate constant relates the rate of contaminant elimi- nation to the concentration of the contaminant in the organism. If the initial concentration in the organism is negligible, Ce = R/kct, where R is the (constant) rate of contaminant intake. Modifications of this deterministic model may incorporate multiple sources, multiple elimination components, growth, trophic efficiency and inter- nal exchange. However, the enhanced realism as- sociated with such complex models can never be freed from a dependency upon the assumptions associated with the simple model, i.e., constant up- take rate, instantaneous mixing within compart- ments, time- or age-independent probability of transition between compartments and a negative exponential depuration process for all compart- ments. Under field conditions, the rate of con- taminant intake (R) may not be constant due to (2) 141 .